While I have not done research in this area myself, I am fascinated
by the
computer graphic technique known as non-photorealistic rendering.
(Admittedly it is a little odd to name a field of study by what it is
not.
Stanislaw Ulam apparently once remarked: "The study of non-linear
physics is like the study of non-elephant biology.") My
list of
links on this topic began to outgrow its place on my bookmarks list, so
I
created this page to give them a home, and perhaps to help other people
interested in the field.

To better describe the kinds of techniques listed here, and to
define the
informal taxonomy used on this page, it is helpful to note that
techniques
for stylized depiction can be classified along the axis from
interactive to
fully automatic, and that there are three distinct types of input for
these
stylized depiction processes:

In computer graphics, photorealistic rendering attempts to
make
artificial images of simulated 3d environments that look "just like the
real
world." So non-photorealistic rendering (NPR) is any technique that
produces
images of simulated 3d world in a style other than realism.
Often
these styles are reminiscent of paintings (painterly rendering),
or of
various other styles of artistic illustration (sketch, pen and ink,
etching,
lithograph, ...) Of particular commercial interest are techniques that
can
render 3d scenes in styles which match the "look" of traditionally
animated
films. Often called 'toon shading, these techniques allow for
seamless combination of 3d elements with traditional cel animation.

Another important application of non-photorealistic rendering is to
help
the user understand that a depiction is only approximate.
Psychologically,
photorealistic rendering seems to imply an exactness and perfection
which may
overstate the fidelity of the simulated scene to a real object. The Piranesi system
(mentioned below) has proved more useful than photorealistic rendering
in an
application that pre-visualizes kitchen remodeling.
The
Teddy modeler (see below) uses a sketchy rendering style to suggest
the
approximate 3D shapes it has inferred from the user's 2d drawing.

Also listed here are some image processing techniques which
can
transform an image into a style that suggests a painting or other
artistic
styles. The images can be photographic or from any other source. Some
of
the techniques have the ability to process a series of images, as from
a
video or film camera, and to produce a stylized image that remains
stable
from frame to frame, allowing the creation of something like an
"animated
painting."

This page also includes: (1) some computational techniques that
carefully
simulate traditional artistic media such as watercolor and ink
painting, (2)
some references to work on non-traditional perspective common
to
traditional illustration but rare in computer graphics, (3) techniques
for
finding and using silhouettes of 3D objects, (4) non-realistic
techniques
designed for real time or interactive rates, and (5) non-traditional
halftones
and screenings.

Finally it should be noted that any claim that an automatic process
can
produce "art" or a "painting" should be regarded as suspect. Making
art is a creative and thoughtful process. It may even be
uniquely human. The possibility of artificial creativity, let
alone
artificial intelligence, are open research questions. None of
the
techniques described here are candidates for true artificial
creativity. I
have tried to use neutral language ("...suggests a
painterly
style...") on this page to skirt the distinction between art
and procedural techniques for stylized depiction.

(Note: items marked [new] have been
added in the three months prior to the time of the "last update"
indicated at the bottom of this page.)

EVA Expressive
Renderer (1999) by Dave
Gordon
is a unique rendering framework utilizing a C++ based shading language
and
provides a host of 2D and non-photorealistic effects based on the
concept of a mark generator. Take a look at the amazing imagery
in the EVA gallery.

Non-Invasive,
Interactive, Stylized Rendering (2001) by Alex Mohr and Michael Gleicher
describes a
technique for taking an existing OpenGL application and changing it to
use new
visual stylizations. This is done "non-invasively": without modifing
the
application's source code. Instead the OpenGL calls are intercepted and
reinterpreted to produce stylized portrayal. The web page includes
animation,
additional figures and the full paper is available(PDF, 0.04 MB).

Artistic
Rendering of Portrait Photographs thesis research by Eric
Wong: preliminary samples of photographic portrait images processed
into
what looks like pencil drawing. (Note: Eric
graduated, Cornell removed his pages. When he sets up a new web site
this link will be
updated. 12-7-99)

Paint
By Relaxation (2001) by Aaron
Hertzmann describes painterly image processing of still and moving
images.
A user-defined energy function controls the stylization. The
output
image is generated by searching for a collection of brush strokes
with
minimal energy. This approach yields good frame to frame coherence for
moving
images. Full paper is available in several lengths and formats
including this
CGI 2001
paper (PDF 17 MB).

Simulating
Decorative Mosaics (2001) by
Alejo Hausner converts input images to mosaics composed of shaped
tiles.
Uses a relaxation technique to minimize the visible grout (the
area
between tiles) while aligning tiles with user-specified feature
boundaries.
The full paper is
available (PDF 1.7 MB)

Pastel-Like Rendering Considering the Properties of Pigments and
the
Support Medium (2002) by Kyoko Murakami
and Reiji
Tsuruno. "A new NPR technique that reproduces pastel drawing-like
textures by
focusing especially on the attributes of pastel pigments." This
SIGGRAPH
sketch is apparently not online, but there is a PDF article of the same
name in
this special issue.

Sketchy Rendering
(2002) by John Haddon
briefly
describes
software to "reproduce the appearance of line drawings in both pencil
and ink,
along with other effects such as the bleeding of ink in water and the
application of pastel to paper." Full
text (PDF, 1.5MB) is available online.

Contour
Rendering (1999) by Per
H. Christensen, describes the Mental Ray contour shader.
Unfortunately
the illustrations from the hardcopy edition of Computer Graphics
33(1)
are not included on this web page.

Style Machines
(2000)
by Matthew Brand and Aaron Hertzmann.
Stylistic
motion synthesis by learning motion patterns from motion capture (or
other
sources). New motions can be generated from learned styles, or
combinations
of them. Full paper is available(PDF 1.4 MB).

Image-Guided
Streamline Placement (1996) by Greg Turk
and David Banks.
"Accurate control of
streamline density is key to producing ... visualization of
2-dimensional
vector fields. We introduce a technique that uses an energy function to
guide
the placement of streamlines at a specified density..."

The Toon Shading process was developed by Tad
Gielow and MJ Turner. Tad says: "The process involved two renders, one
for color (via Wavefront's Render) and one for plotter output (via
Wavefront plot). The plotter output render was suitable for driving a
pen plotter to create artwork that integrated into the traditional Ink
and Paint process (used on The
Great Mouse Detective, Oliver
and Company, The Little
Mermaid, and a number of shorts). We took the plotter output and
wrote a script to drive a Pixar Image
Computer to draw the lines. The lines images was then composited
over the color render."

Burning Love (1988) short by Carlos Arquelos et al.
at PDI. Plese contact me
if you know any details.

DNA
(2002, QuickTime, 3.8MB, 60 seconds) cel shaded TV spot
for Volkswagen
by Psyop. Video is available on
this page
of VW Commeercials, at
Ads.com and at Psyop's
gallery. Their site also has two other NPR VW spots: Sheep
(MPEG, 7.8MB) and Cones
(MPEG, 3.5MB).